In direct laser engraving for the production of flexographic printing plates, a printing relief is engraved directly into a relief-forming layer suitable for this purpose with the use of a laser or of a laser system. The layer is decomposed in the areas in which the laser beam is incident on it and is removed substantially in the form of dusts, gases, vapors or aerosols. A development step as in the case of conventional processes—thermally or by means of washout agents—is not required.
Although the engraving of rubber printing cylinders by means of lasers has in principle been known since the 1960s, laser engraving has acquired broader economic interest only in recent years with the arrival of improved laser systems. The improvements of the laser systems include in particular better focusability of the laser beam, higher power and computer-controlled beam modulation.
With the introduction of new, more efficient laser systems, however, the question regarding particularly suitable materials for laser-engravable flexographic printing plates is also becoming more and more important. Particularly in the engraving of high-resolution printing plates or very fine relief elements, problems now occur which played no role at all in the past because laser systems in any case did not permit the engraving of very fine structures. Improved laser systems thus lead to new requirements with respect to the material.
In direct laser engraving, it should be noted in particular that the relief-forming layer which is engraved by means of the laser also forms the subsequent printing surface. All defects which occur during the engraving are thus also visible on printing. In direct laser engraving, in particular the edges of the relief elements must therefore be formed particularly precisely in order to obtain a crisp printing image. Frayed edges or beads of molten material around relief elements, i.e. melt edges, have a considerable adverse effect on the printed image. Of course, the finer the desired relief elements, the more important are these factors. EP-B 640 043 and EP-B 640 044 have proposed amplifying laser-engravable flexographic printing elements and if necessary adding materials which absorb laser radiation for improving the sensitivity. The use of carbon black is also proposed without this being specified more precisely.
Carbon black is not a defined chemical compound; instead, there is a very large number of different carbon blacks which differ with regard to preparation process, particle size, specific surface area or surface properties and which accordingly also have a very wide range of chemical and physical properties. For further details, reference may be made, for example, to H. Ferch, Pigmentruβe, edited by U. Zorll, Vincentz Verlag, Hanover, 1995. Carbon blacks are frequently characterized by the specific surface area, for example determined by the BET method, and the structure. A skilled worker in the area of carbon blacks understands structure as meaning the linkage of the primary particles to form aggregates. The structure is frequently determined by means of the dibutyl phthalate (DBP) adsorption. The higher the DBP adsorption, the higher the structure.
The conductivity carbon blacks form a special class of carbon blacks. In general, carbon blacks having a DBP adsorption of more than 110 ml/100 g and a relatively high specific surface are referred to as conductivity carbon blacks (Ferch loc.cit., p 82). Conductivity carbon blacks are usually used for making nonconductive materials electrically conductive with the addition of a very small amount.
The use of carbon black in laser-engravable flexographic printing elements has also been described by EP-A 1 080 883, WO 02/16134, WO 02/54154 or WO 02/083418. Said publications, however, disclose not conductivity carbon blacks but carbon blacks having a relatively small specific surface area and small DBP number.
EP-A 1 262 315 and EP-A 1 262 316 disclose a process and a laser system for the production of flexographic printing plates. The laser system described operates with a plurality of laser beams which may have different power and/or wavelength, and by means of which the surface regions of the printing plate and deeper regions can each be processed separately. Reference is made to the possibility of making the surface of the flexographic printing element used different to the regions located underneath. However, the documents contain no proposals at all with regard to a specific chemical composition for the surface or the regions located underneath.
It is an object of the present invention to provide a one-layer or multilayer laser-engravable flexographic printing element which also permits the engraving of fine relief elements with high precision without the occurrence of melt edges. It should be suitable in particular for engraving using modern multibeam laser systems.
We have found, surprisingly, that this object is achieved by the use of conductivity carbon blacks of the type defined at the outset. The flexographic printing elements can be engraved with high resolution without melt edges and other adverse effects being observed. The result was surprising in particular because said carbon blacks are by no means those which have the highest sensitivity to laser radiation.
Accordingly, flexographic printing elements for the production of flexographic printing plates by means of laser engraving have been found, which at least comprise, arranged one on top of the other,                a dimensionally stable substrate and        at least one relief-forming, crosslinked elastomeric layer (A) having a thickness of from 0.05 to 7 mm, obtainable by crosslinking a layer which comprises at least one elastomeric binder (a1), a substance (a2) absorbing laser radiation and components for crosslinking (a3),wherein the substance absorbing laser radiation is a conductivity carbon black having a specific surface area of at least 150 m2/g and a DBP number of at least 150 ml 100 g.        